As speed and data volume of radio communication increases, there has arisen an increasing demand for accurate linearity of an amplifier, in particular, a power amplifier, but in the power amplifier, input power is in a trade-off relation with linearity and efficiency. That is, when an input signal is small, linearity and efficiency are high, and when the input signal is large, linearity and efficiency are low. Therefore, there have been proposed techniques to compensate nonlinearity of a power amplifier so as to achieve high linearity and high efficiency even when an input signal is large.
As techniques to compensate nonlinearity of a power amplifier, there have been known techniques such as a negative feedback method, a feed-forward method, an analog pre-distortion method in which the nonlinearity is compensated in an analog signal stage. In recent years, a digital pre-distortion method has been drawing attention which estimates a characteristic of a power amplifier by using digital signal processing and gives inverse distortion to a digital signal in advance (see, for example, JP-A 2001-36353 (KOKAI)).
A pre-distortion method disclosed in JP-A 2001-36353 (KOKAI) combines the compensation of an amplitude characteristic and the compensation of a phase characteristic. Specifically, as the compensation of the amplitude characteristic, amplitudes Vi, Vq of output baseband signals I, Q of roll-off filters are first inputted to polynomial operation units, which then perform a polynomial operation to compensate an amplitude-amplitude characteristic (hereinafter, it will be referred to as an “amplitude characteristic”, and similarly, an “amplitude-phase characteristic” will be referred to as a “phase characteristic”). Then, as the compensation of the phase characteristic, a power calculator calculates power (square sum) by using signals g(Vi), g(Vq) resulting from the compensation of the amplitude characteristic, and based on the calculation result, a compensation value is read from a table storing an inverse characteristic of the phase characteristic of the power amplifier, and the compensation value and the signals resulting from the compensation of the amplitude characteristic are multiplied. With a small-capacity table memory and a small-scale arithmetic logic, this procedure is executed, thereby realizing the compensation of nonlinearity of the power amplifier.
Generally, the operation state of a power amplifier changes depending on a surrounding environment and a characteristic of a signal to be amplified, and therefore, according to such a change, a compensation value or a compensation coefficient used for compensating the amplitude characteristic and the phase characteristic need to be updated in order to ensure linearity of the power amplifier. However, since the update of the compensation value or the compensation coefficient requires power consumption, there is a demand for a technique to realize both linearity of the power amplifier and reduced power consumption.    Patent Citation 1: Patent 2001-036353